1. Field of the Invention
The present invention relates to a method and apparatus for determining an absolute angle and torque with an optical detection module, in which the absolute angle and the torque are determined on the basis of measurements including marked waveforms at both ends of a torsion bar, relative rotation angles at both ends of the torsion bar, and revolutions per minute of the torsion bar measured with the aid of a revolution number measurement unit.
2. Description of the Related Art
U.S. Pat. Nos. 5,930,905 and 6,466,889B1 disclosed methods for determining an absolute steering angle in dependence of rotation of a steering shaft. According to the disclosure, an absolute angle of the steering shaft can be determined based on measurements of rotation angle of two rotatable bodies (i.e., a first rotatable body and a second rotatable body) that rotate at a certain rotation ratio with respect to the shaft.
Suppose that absolute rotation angles of the first rotatable body and the second rotatable body are ψ=ψ′+iΩ and θ=θ′+jΩ, respectively (wherein, ‘Ω’ denotes a measurement range of an angular sensor measuring the ψ′ and the θ′; ‘i’ is an integer indicating number of times when the ψ is greater than Ω(ψ>Ω); and ‘j’ denotes a frequency of the second rotatable body). Both related art methods introduced an idea of determining an absolute steering angle φ of a steering shaft through a designated calculation on the measurements of ψ′ and θ′.
Particularly, according to the method disclosed in U.S. Pat. No. 5,930,905, the measurements of ψ′ and θ′ are substituted into a following equation (1) that is derived from a geometric relation between ψ, θ, and φ, and through rounding off, an integer (or a whole number) k is obtained.k=[(m+1)Θ′−mψ′]/ψ  Equation (1)
The value k, ψ′ and θ′ are substituted into an equation (2) to obtain φ.φ=[mψ′+(m+1)Θ−(2m+1)kΩ]/2n  Equation (2)
In Equation (2), ‘m’ denotes number of gear teeth of the first rotatable body; ‘m+1’ denotes number of gear teeth of the second rotatable body; and ‘n’ denotes number of gear teeth formed on a steering shaft, in which the first and second rotatable bodies are in gear to the steering shaft.
U.S. Pat. No. 6,466,889B1, on the other hand, disclosed a method for determining a steering angle φ out of ‘i’ that is obtained directly from a relation between the ‘i’ value of the first rotatable body (or the second rotatable body) and the difference of absolute rotation angle ψ−θ of two rotatable bodies. More specifically, if the difference between the measurements of ψ′ and θ′, i.e., ψ′−θ′, is negative, Ω is added to ψ−θ, but if not, the value of ψ−θ is maintained. Then, from the relation between ψ−θ and ‘i’, the value ‘i’ is calculated. Here, ψ is calculated by using known values ψ′ and ‘i’. In this manner, the absolute steering angle φ. Since the measurement range of the angle sensor is Ω, if the steering shaft is rotated to the maximum and thus, the value ‘i’ equals to ‘k1’, the difference of rotation angles ψ−θ should be equal to or less than Ω(ψ−θ≦Ω) (in case of U.S. Pat. No. 6,466,889B1, ψ−θ was set to be equal to Ω). That is, the difference of rotation angles ψ−θ continuously changes from 0° to Ω until the steering shaft rotates to the maximum, and the value ‘i’ changes step by step from 0 to ‘k1’.
What is assumed in U.S. Pat. No. 6,466,889B1 is that the difference of rotation angles ψ−θ and the value ‘i’ are in linearly proportional relation to each other (i.e., when the difference of rotation angles ψ−θ continuously changes from 0° to Ω, the value ‘i’ continuously changes from 0 to ‘k1’). In effect, the value ‘i’ is obtained by rounding the multiplication result of k1/Ω and the ψ−θ to the nearest whole number that is less than the multiplication result. For instance, if the multiplication result of k1/Ω and the ψ−θ is 5.9, the value ‘i’ becomes 5.
However, the method disclosed in U.S. Pat. No. 6,466,889B1 has a limitation that a maximum value of the ψ−θ should not be greater than Ω. This means that ‘i−j’ has to be 0 or 1 all the time, and should not be greater than 2.
Meanwhile, a torque detection device for use in automobiles typically uses a torsion bar to obtain torque. By measuring how much the torsion bar is twisted, and then the angle difference at both ends of the torsion bar, it becomes possible to calculate torque.
There are several methods for measuring the amount of twist in the torsion bar. For example, U.S. Pat. No. 6,285,024 disclosed a method for measuring an angle within 360° by transmitting a light to a rotatable body, U.S. Pat. No. 6,350,044 disclosed an analysis of a reflection pattern of a light by marking irregular, regular waveforms on a rotatable body, and U.S. Pat. No. 6,578,437 disclosed a method for determining an absolute angle and torque based on a measurement of rotation angle under the presence of a magnetic field.
Particularly, the optical detection module disclosed in U.S. Pat. No. 6,450,044 was used to measure an absolute rotation angle and torque of an electric power steering. However, because the measurement range of an angle sensor thereof was limited to 360°, another method like gear ratio reduction had to be used also. In result, measurement accuracy had deteriorated, and even a measurement device itself had a very complicated structure and shape for use.
Although a magnetic field sensor made it possible to measure an angle greater than 360°, the electric power steering (EPS) located near to a motor was too easily exposed to a magnetic field, and this increased possibility of accident occurrence.